Trees/plants/rocks spam for my game terrain
If you intend to export a whole terrain to unity (or any other RT 3D game engine), it would be wise to create your terrain, the trees, rocks and so on as independent objects, then import them into unity and let unity's distribution method do the work. 

The reason for this is that unity works with instances of objects, while Blender would produce actual geometry or - if you do not intend to join your terrain into one object - many objects with possible redundancy of materials. That means more draw calls which lead to a heavy load on the engine. 

Besides from that, you would still need to create collision meshes, which means even more redundant data or, if you chose to join all objects into one mesh with submeshes, it would be nearly impossible to generate the collision meshes automatically.